The TREXIO file format and library

Within the "Targeting REal chemical accuracy at the eXascale" (TREX) European center of excellence, we are building a file format and library to help inter-operability between quantum chemistry codes, and also to help the reproducibility of calculations. This is a tedious task because the codes have different conventions such as the normalization of the
basis, the ordering of atomic orbitals, the phase factors in the CI coefficients, etc...
We propose a file format in which these conventions are well defined, such that the users don't need to know which code produced the file to be able to use the data inside it. The access to the data in the file is made via TREXIO, a C-compatible library which has a Fortran and a Python binding. In addition to the storage of all the wave function parameters (atomic basis set, molecular orbitals, CI coefficients, ...), the TREXIO library also allows the storage of one- and two-electron integrals as well as one- and two-body reduced density matrices.